More boys are born in southern Europe than the north, and vice versa in North America - but scientists do not know why.

A study published in the British Medical Journal looked at whether latitude had an effect on sex ratios.

But experts commenting on the study said though latitude could have an effect, the area was extremely complicated and there could be multiple reasons for different gender patterns in different countries.

In the study, carried out by researchers at St Luke's Hospital, Guardamangia, Malta, Europe was divided into three bands.

I don't think you could just use latitude

Dr Ian Hardy, University of Nottingham

They compared southern countries, such as Malta, Italy and Spain (latitude 35 to 40°) with central, including Austria , France and the UK (40-55°) and Nordic countries, such as Denmark, Iceland and Sweden (over 55°).

North America was divided into Mexico (under 30°), the United States (30-50°) and Canada (over 50°).

Trends

Data from the World Health Organisation was used to look at the patterns of births over the last 50 years.

It was found significantly more boys were born in southern European countries than in central Europe or Nordic countries.

In the UK, the figures were 20,441,737 male births and 19,330,020 female
births.

But in north America the reverse pattern was seen, with a low male to female ratio in Mexico, a higher ratio in the US, and a higher ratio still in Canada.

The male to female ratio for the total number of births in both central and southern Europe combined was significantly higher than for the US, even though both land masses spanned the same
latitudes.

Writing in the BMJ, the research team's leader, Dr Victor Grech, a paediatric consultant at St Luke's Hospital said: "We are unable to explain these findings, which do not support a temperature related effect."

Dr Ian Hardy, from the University of Nottingham, who has edited a book on sex ratios, told BBC News Online latitude could be a factor: "It certainly isn't beyond all possibility that there is some sex ratio effect."

But he added there were a lot of theories in the field of research: "I don't think you could just use latitude."